This invention relates to semiconductor photodetector devices and more particularly to depletion mode thin film semiconductor photodetectors, e.g., thin film transistors (TFT's) or photodiodes, especially suited for use in large area image sensor arrays.
Because of its ease of disposition on transparent substrates over a large area and ease to obtain reproducible transistor operations, crystallized silicon TFT's show great potential in applications including large area displays, photodetectors, image sensors and integrated optics devices.
Because of its photosensitivity, silicon has been employed in photosensors in the configurations of photodiodes, phototransistors and charge-coupled devices. Ordinarily, these configurations are fabricated on bulk single-crystalline silicon substrates. The overall size of such configurations is limited by the size of the silicon wafer. The use of light collection optics for reading a large image area requires a bulky sensing system.
Because of the ease of deposition of amorphous silicon (a-Si:H) thin films on transparent substrates of arbitrarily large size, these thin films have been employed in large area image sensor arrays for contact document input scanners. See, for example, U.S. Pat. No. 4,419,696. However, because of the low carrier mobilities in a-Si:H, the photosensitivity and the speed are, in general, found to be poorer than compared to sensors fabricated in bulk crystalline silicon (c-Si).
Recent progress in laser crystallization have enabled formation of defect free, single crystalline silicon thin films on bulk amorphous substrates for formation of high performance TFT's to permit applications to direct view large area electronic devices such as flat panel displays and image sensors on transparent substrates. See for example, U.S. Pat. No. 4,409,724. Recent demonstration of TFT shift registers also promises to broaden the field of application from local switching of individual transducers to include logic circuits having sufficiently high speed for data rates often encountered in image processing. See, for example, A. Chiang et al., in "NMOS Logic Circuits in Laser Crystallized Silicon on Quartz", Proceedings of the 1983 Materials Research Society, Boston, Mass., November, 1983, and published in the book, Energy Beam-Solid Interactions & Transient Thermal Processing, John C. C. Fann and N. M. Johnson, editors Vol. 23, pp. 551-558 (Elsevier, N.Y., 1984).
Crystallization of silicon thin films on insulating substrates, e.g., fused quartz, in patterned and encapsulated configuration by a scanned CW laser have been well documented in the art. See, for example, U.S. Pat. No. 4,330,363 and the book, Laser & Electron-Beam Solid Interactions & Materials Processing, J. F. Gibbons et al, editors, Vol. 1, in particular the article of N. M. Johnson et al at pages 463-470 entitled "Processing and Properties of CW Laser-Recrystallized Silicon Films on Amorphous Substrates" (Elsevier, N.Y., 1981). The recent technological breakthrough involves using a solidification front tilted from the path of the molten zone of the scanned CW laser. Structural defects can be greatly reduced or even eliminated by a lateral defect-sinking thus permitting formation of very high percentage of (100)-textured single crystalline silicon islands with &lt;100&gt; in-plane orientations. See the Proceedings of the 1983 Materials Research Society Meeting, supra and L. Fennell et al., "Defect Reduction by Titled Zone Crystallization of Patterned Silicon Films on Fused Silica, both soon to be published in the book, Energy Beam-Solid Interactions & Transient Thermal Processing, John C. C. Fann and N. M. Johnson, editors Vol. 23, pp. 403-408 (Elsevier, N.Y., 1984).
These advances in the technology of laser crystallized silicon thin films on insulating substrates have produced numerous novel NMOS and CMOS thin film electronic devices with performances comparable or superior to that of bulk devices. However, the photosensitivity demonstrated in enhancement mode TFT's has not been sufficiently great to provide a desired level of S/N ratio and dynamic range for contact input scanner applications such as high speed, high resolution in compact electronic copiers or printers or facsimile machines.